You can share folders, it gives a similar URL to when you share files. People can then view the folder, and access any files or subfolder within it. You can also add or remove stuff to the folder and people can still use the same link, it'll update for them.
Also, MediaFire says that it'll keep your files as long as 1) they're being downloaded and/or 2) you keep logging in. I don't think there's a set time for inactivity, that is it might be different for each account, but they will send you 4 emails notifying of deletion before they actually delete anything, so you can just login when you get that.
[SPOILER=Full quote from MediaFire]
Also, MediaFire says that it'll keep your files as long as 1) they're being downloaded and/or 2) you keep logging in. I don't think there's a set time for inactivity, that is it might be different for each account, but they will send you 4 emails notifying of deletion before they actually delete anything, so you can just login when you get that.
[SPOILER=Full quote from MediaFire]
MediaFire FAQ Wrote:For MediaPro subscribers, your uploaded files will be stored forever as long as your Pro account is active. For free accounts, there is currently no time limit on how long uploaded files will be stored as long as you access your account (i.e. login to your account) or your files are being accessed (i.e. downloaded). Every account is different and different accounts may become inactive at different times.[/SPOILER]
Users receive at least 4 notification emails that files are pending deletion before any deletion actually occurs. Logging into your account is sufficient to prevent deletions.